Suppose if some one went Umrah and completed all the rites of it, except cutting of hair. When he reach to his station than he remember that he didn't cut the hair. Now what is the solution for it?

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

The preponderant opinion of Muslim scholars is that shaving or shortening one's hair is an integral part of Hajj and Umrah. So, it is forbidden to commit any acts restricted by Ihram before shaving or cutting one's hair short if he is male, or cutting her hair short if she is female.
The one who forgot to shave or cut his hair short or did not know it is obligatory and after completing the rituals he remembers or knows the ruling only two situations apply to such a person:
A) If he has already committed any restricted acts of Ihram he should immediately comply after knowing or remembering the rulings. Therefore, if a man has worn normal dress such as garment or pants etc., he should remove it and wear the dress of Ihram and shave or cut his hair short. The violations of Ihram would be excused due to his forgetfulness or ignorance as preferred by some scholars.
B) If one did not violate any restrictions of Ihram. Such a person should shave or cut hair after knowing or remembering the rulings.
Allah knows best.